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202 (2008) 4369–4373www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Warm Spraying: An improved spray process to deposit novel coatings

Jin Kawakita a,⁎, Hiroshi Katanoda b, Makoto Watanabe a, Kensuke Yokoyama a, Seiji Kuroda a

a National Institute for Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japanb Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan

Available online 9 April 2008

Abstract

Warm Spray is an atmospheric coating process through continuous impact and deposition of solid particles heated and accelerated by asupersonic jet controlled between 800~1900 K and 900~1600 m s−1. This paper introduces successful fabrication of dense and less-oxidizedmetallic titanium (Ti) coatings by Warm Spray and clarification of phenomena occurring upon the spray process. Temperature and velocity of anin-flight Ti particle were compared between measurement by the diagnostic instrument and calculation based on the fluid dynamics simulation.Deformation behaviour of particle from impact to deposition was analyzed through the finite element method (FEM). Densification of stackingparticles was attained by applying bi-modal size distribution to the feedstock Ti powder. Qualitative restriction of changes in chemicalcomposition of Ti coating obtained was demonstrated by elemental analysis and by calculation based on the oxidation model. Warm Sprayenables various materials to fabricate coatings without thermal deterioration of the original characteristics such as purity and crystallographicphase.© 2008 Elsevier B.V. All rights reserved.

Keyword: Supersonic thermal spraying; Titanium coatings

1. Introduction

It is extremely advantageous to be able to coat variousmaterials in the open atmosphere in order to improve theoperation efficiency, to remove the limitation in the targetdimension, In general, conventional thermal spray methodmade feedstock materials to adhere by melting, leading to takeadvantage of comparatively high adhesion strength without anybinder. In the technique, especially realized at atmosphericpressure, it has been difficult to keep the original state of thematerials because of changes in chemical composition, phasetransition and textural change of the materials. A concept toovercome the above issue is to suppress the temperature of thesupplied particles to lower as much as possible with theirimpact velocity to the target substrate higher than the criticalspeed specific for deposition of each material. Furthermore, in-flight particles are impacted and deposited to fabricate coatings.To realize this concept, high-velocity oxy fuel (HVOF)spraying [1] and Cold Spray process [2] have been developed

⁎ Corresponding author. Tel.: +81 29 859 2445; fax: +81 29 859 2401.E-mail address: KAWAKITA.Jin@nims.go.jp (J. Kawakita).

0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2008.04.011

and industrialized in recent 15 years. Browning invented theprinciple of HVOF spraying [1], which accelerated and heatedthe feedstock material supplied into the supersonic jet flamemade from fuel and oxygen or air. HVOF is often used forcoating of cermet materials alternative to hard chrome platingbecause of environmental regulation [3]. In HVOF, temperaturerange of impact particle is around 1500~2500 K and thermaldegradation of the materials is included. Furthermore, it isdifficult to control the jet temperature and velocity indepen-dently and therefore the relation between specific sprayparameter and coating characteristic such as fuel flow rateand coating porosity, respectively leading to consume muchtime to optimize the spray parameters. On the other hand,Papyrin found the phenomenon of metal deposition as a hint ofCold Spray process, which accelerates fine powder by asupersonic jet derived from adiabatic expansion of heated gas[2]. Most reports are related to metal and alloy with plasticdeformation property [4]. Cold Spray has a difficulty inincreasing the temperature of impact particle above 800 K. Thetemperature of melting and phase transformation of mostinorganic materials is around 800~1500 K. In addition,cohesion and densification are controlled by the mechanicalproperties of the materials depending on the temperature.

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Accordingly, it is meaningful to develop a spray process tocontrol the temperature of impact particle between 800 and1500 K in order to realize coating of the materials keeping theoriginal characteristics. Warm Spray is one of the technologiesto achieve this concept [5]. It's a process that the temperatureof the supersonic combustion jet composed of the fuel(kerosene) and oxygen is controlled by introduction of theinert gas (nitrogen) and that feedstock powder is supplied intothe resulting supersonic jet controlled between 800~1900 Kand 900~1600 m s−1 [6] and is heated and accelerated, leadingto be impacted and deposited continuously upon the targetsubstrate.

So far, even the highly active metals such as titaniumand metallic glass metal have been successful in fabricatingthe coatings with high purity and high density by thistechnique [5,7]. Moreover, metal oxides such as photo-catalytic TiO2 could be deposited keeping with the nano-structure and the original phase, leading to exert functionalperformance [8,9].

It is believed that the state evaluation and the phenomenonclarification of each step in the Warm Spray process arenecessary from a fundamental academic viewpoint, to shortentime until the spray conditions are optimized and to improve afurther apparatus characteristic. Titanium (Ti) was chosen to bethe coating material in this paper. This is because Ti hasseveral attractive properties such as superb corrosion resis-tance, inertness in human body, light weight, high specificstrength and low paramagnetism. Coating of Ti is highlyexpected to have various applications as the structural stuff,heat exchanging plants, and surgical implants, etc. On theother hand, Ti is a very reactive metal at high temperaturesbecause of its strong affinity with gases such as oxygen,nitrogen and hydrogen. Moreover, since Ti is composed ofsingle element it was expected easy to estimate the change inchemical composition and in crystallographic phase. Inaddition, the melting point (1941 K) is high and the yieldstress is decreased to 110 MPa around 800 K. Therefore, Ti isone of the suitable materials to study phenomenon such asadhesion by instantaneous melting and sheer instability nearparticle surface upon impact and densification by plasticdeformation of impact particle.

In this paper, the following research topics were investigatedwith several experimental evaluation methods and modelcalculation and simulation;

1) the state of in-flight Ti particle upon Warm Spray process;2) Ti particle deposition mechanism and coating density;3) purity of Ti coatings.

2. Experimental procedures

2.1. Spray condition

The primary parameters of Warm Spray were flow rates ofkerosene as fuel, oxygen gas and nitrogen gas similarly asreported previously [5]. Nitrogen was set between 0.5 and2.0 m3 min−1. The spray distance between the barrel exit and

the target position was also varied from 100 mm to 280 mm inthis paper.

Ti powder with particle size under 45 μm was used as thebase powder (Sumitomo Titanium, TILOP). Bigger Ti powderwith the particle size of 63~90 μm or 90~150 μm was mixedwith the base powder in the content range of 0~50 mass% in thefeedstock. The chemical composition of Ti powder is Fe 0.031,O 0.141, C 0.007, N 0.011, H 0.007, and Ti balance in mass%.

2.2. Evaluation and measurement

The in-flight velocity of sprayed particles was measured bythe in-flight diagnostic instrument for thermal sprayed particles(TECNAR Co., DPV-2000). Its principle and mechanism weredescribed in detail elsewhere [10]. The system calculates thesurface temperature of the particle from the ratio of radiationenergy at the two wavelengths defined by the two filters and thevelocity of the particle simultaneously by dividing the distancebetween the two slits by the time interval between two radiationpeaks detected when the image of one particle passes in front ofthe slits.

Ti powder was sprayed to fabricate coatings onto thesubstrate of low carbon steel (JIS: SS400) and titanium (JISsecond grade) with the dimensions of 50×100×5 mm. Thesubstrates were blast cleaned with alumina grit and degreased inacetone ultrasonically.

The mercury intrusion porosimeter (Micromeritics, Autop-ore II 9220) was used to measure the pore size distribution of thecoatings. For this method, sprayed Ti coatings with approxi-mately 1 mm thickness were delaminated from the substrate,broken into pieces (approximately 7×7 mm) and used for themeasurement.

The cross section of the Ti coatings was observed by theoptical microscope (Olympus, BX60M). The cross section wasprepared by ordinary metallographic technique, i.e. embeddingthe coated specimen into an epoxy resin, followed by grindingand polishing treatments.

Phases and crystal structure of the coatings were analyzed byan X-ray diffractometer (Rigaku, RINT2500). Oxygen, nitrogenand carbon content in the coatings were analyzed by the inertgas fusion method. Specimens for this measurement wereprepared with the same technique as for the mercury intrusionporosimetry.

For the corrosion tests, Ti coated specimens with thethickness of 400 μm on the substrate were cut into pieces in asize of 25 mm square and were cleaned ultrasonically in acetoneand ion-exchanged water, repeatedly. The stainless lead wasconnected to the back surface of the substrate plate. The sprayedarea of 200 mm2 was left exposed and the rest of the specimensurface was insulated with silicone resin. The Ti coatedspecimen onto the substrate of carbon steel (JIS SS400) wasimmersed into the aerated artificial seawater at 300 K at pH8.3for 3 days. To obtain polarization curve of the Ti coatings, theelectrochemical cell with the three-electrode system was used.The sample electrode was prepared as explained above. In thiscase, however, the Ti coated specimen onto the titanium plate(JIS second grade) was used to get rid of the effect of the

Fig. 1. Velocity of in-flight particle as a function of particle position, (a) calculation and (b) observation. Position=0 is the exit of nozzle of Warm Spray apparatus.

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substrate on the polarization behaviour. In addition, the surfaceof the specimens was polished. The counter electrode was aplatinum plate with a dimension of 0.2×100×100 mm. Thereference electrode was the Ag/AgCl electrode in the saturatedKCl solution. The electrolyte was artificial seawater of pH8.3 at300 K and continous nitrogen bubbling was carried out fordeaeration. The electrode potential of the sample was scanned atthe rate of 10 mV s−1 using a potentiostat with a functiongenerator (Hokuto Denko HAB-151).

3. Model calculation and simulation

The gas velocity and temperature inside the spray apparatusand after the barrel exit were simulated based on the gasdynamics modeling [11]. Once the gas flow field was obtained,the velocity and the temperature of spherical Ti particle injectedat the powder feed port were calculated along the axial trajectory.

Deformation behaviour of Ti particle impacting on the Tisubstrate was simulated by the finite element method based onJohnson–Cook plastic deformation model [12].

Ti particle oxidation during the in-flight process wasestimated based on Wagner-type oxidation model [6]. For theoxidation model, it was assumed that the Ti particle was covered

Fig. 2. Temperature of in-flight particle as a function of particle position, (a) calculatio

with an oxide scale and that the inner layer was a solid solutionfor oxygen and titanium.

4. Results and discussion

4.1. State of sprayed particles

Regarding the in-flight particles' velocity and temperature,the calculation through the fluid dynamics simulation and themeasurement with a flight particle state measurement devicewere compared. Fig. 1 shows the calculated and observed valueof in-flight particles with respect to the velocity, respectively.Similar curves by both calculation and observation can be seenat each spray condition except the curves at N2 flow rate of0.5 m3 min−1. The in-flight velocity exceeded twice higher thansupersonic speed, changed in a parabola along with the spraydistance, and was independent on the flow rate of the coolant N2

gas. At N2 flow rate of 0.5 m3 min−1, observation indicatedhigher value than calculation at a certain spray distance. Thismight be due to detection of specific in-flight particles of all theparticles with the size distribution. It is considered that a smallerparticle is oxidized more easily at a higher temperature, leadingto enhancement of radiation by heat of oxidation reaction. In

n and (b) observation. Position=0 is the exit of nozzle of Warm Spray apparatus.

Fig. 3. Results of FEM simulation of Warm Sprayed particle upon impact anddeposition process, (a) profile of Ti particle stuck to Ti substrate and (b) relationbetween calculated temperature and velocity critical for particle deposition.

Fig. 4. Polarisation curves of titanium coatings with different porosity and bulkplate (JIS second grade) in deaerated artificial seawater at 300 K.

4372 J. Kawakita et al. / Surface & Coatings Technology 202 (2008) 4369–4373

addition, the smaller particle can be easier accelerated to highervelocities.

Fig. 2 shows the calculated and observed value of in-flightparticles with respect to the temperature. It was found that theparticle temperature was controllable in a wide range of800~1500 K on calculation. The difference of the resultbetween the calculation and the measurement was caused bythe influence of the assumption for simulation on thecalculation. For instance, oxidation of particles generated theheat of reaction, leading to the increase in surface temperatureof the in-flight particles. The increasing tendency on the spraydistance is considered to support this assumption.

It is important that both the results be matched by reducingsuch a distribution of the actual particles through an experi-mental technique and by taking the experiment result into theassumption for the simulation.

4.2. Particle deposition mechanism and coating density

The impact and deposition phenomenon of one particle inthe coating was simulated by the finite element method. In thepresent paper, the following bonding mechanism was assumed.The kinetic energy of the particle must be released in the shortduration upon impact. Thus, the temperature rises up quickly at

the interface of the impact particle and the target, and can reachthe melting point of the materials, then adhesion to thesubstrate can be reached, as shown in Fig. 3(a) The cal-culations for various temperature and velocity of the impactingparticle provide a kind of deposition map as shown in Fig. 3(b)This map can offer considerable information about actualstacking of the particle. In fact, almost 100% of the depositionefficiency was observed during fabrication of Ti coatings byWarm Spray process. The influence of the particle sizedistribution on the stacking process will be revealed in future.Cohesion can be attained by the different mechanism with theone mentioned above. Therefore, it's important to get moreinformation regarding adhesion behaviour by experimentalmethods.

The open-porosity of the coatings prepared under the variousspray conditions was varied from 1 to 14 vol.% [5]. The crosssection indicated that a highly dense coating could be obtained.Further densification of Ti coatings was achieved by bi-modalsize distribution of feedstock powder upon Warm Spraying.When 1% of bigger Ti particles were mixed with the usualfeedstock powder under 45 μm, the coating porosity wasdecreased from 2.3 vol.% to 0.8 vol.% simultaneously with thelow oxygen content of 0.26 mass%, which was comparable tothe level of feedstock powder. This densification was alsoclearly observed in comparison of appearance of coating surfaceafter immersion test. If there are connected pores through thecoating, the red–brown corrosion products on the coatingsurface are derived from corrosion of the steel substrate bypenetration of the artificial seawater. In fact, few numbers ofcorrosion products were observed on the coating surface,corresponding to the higher packing density of the coating. Thisdensification is caused by the balance of the enhancement of thepeening effect by big particles and of optimization of the fillingrate of the big and small particles.

Fig. 4 compares polarization curves of the coated specimenson titanium substrate. Ti is a material to have a risk of crevicecorrosion. Therefore, localized corrosion took place onboundaries between particles in the Ti coating prepared throughthe stacking process of particles. Current oscillation on the

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polarization curves corresponded to occurrence of suchcorrosion phenomenon. After the polarization test, the densercoating showed no corrosion spots between the particles'boundaries, which were observed for the less dense coating.Densification enabled corrosion resistance to be improved fromthe result of suppression of current oscillation.

4.3. Purity of coatings

The primary chemical reaction of the flight particle in theWarm Spray process is the oxidation. The oxygen content wassuppressed to significantly low values in comparison with thevalues for the coatings fabricated with other atmospherictechniques [5]. Nitrogen was also expected to be taken up in theTi coatings because the Ti feedstock powder was supplied to thehot jet containing nitrogen gas. The result of the chemicalanalysis showed that the nitrogen content of the coatings wasone tenth smaller than the oxygen content under a certain spraycondition. The chemical composition of the coating wascomparable with that of the feedstock powder as N2 flow rateover 1.5 m3 min−1. Moreover, it was found that the observedvalues of the oxygen content of the coatings were in accordancewith the calculated ones within 1 mass% in difference [6].

From the XRD measurement, the relative intensity ofdiffraction lines attributed to titanium mono oxide, TiO isdecreased with the nitrogen flow rate, i.e. the drop of thetemperature of the impact particle [5]. At N2 flow rate over1.0 m3 min−1, no obvious lines to TiO were observed.

It is summarized that time duration in the high temperaturerange is too short to oxidize nitride and the in-flight particles toa high degree although thermodynamic reactivity of titaniumwith oxygen and nitrogen gas is high even upon Warm Sprayprocess.

5. Conclusions

By using metallic titanium, the state of in-flight particles ineach step in the Warm Spray process was estimated and thedeposition mechanism of the particles and reactions withsurrounding gas were elucidated.

Fabrication of dense and pure Ti coatings was realized inthe air by Warm Spray, which can control the impact particles

to the softening temperature with the velocity as twice as thesupersonic speed.

Further experimental study is necessary to elucidate thestacking mechanism of particles and to agree the experimentalresults with calculation.

A future target is to make the simulation program that canconsistently analyze all processes from a powder into thecoating.

Warm Spray is applicable to other kind of materials, whichhave comparatively high reactivity and require large energy fordeposition.

Acknowledgements

Mr. M. Komatsu and Mr. N. Kakeya are greatly acknowl-edged for their skilful operation of HVOF spraying equipmentand metallographic preparation of obtained samples. We aregrateful to the NIMS Materials Analysis Station for the gasanalysis in the coatings.

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