treatment of metal surface by atmospheric microwave plasma jet

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 4, AUGUST 2006 1241

Treatment of Metal Surface by AtmosphericMicrowave Plasma Jet

Dong H. Shin, Chan U. Bang, Jong H. Kim, Yong C. Hong, Han S. Uhm, Senior Member, IEEE,Dae K. Park, and Ki H. Kim

Abstract—Atmospheric microwave plasma jets generated in Argas or Ar/O2 and Ar/H2 gas mixtures were applied to a surfacemodification of metal. The nozzle-type microwave plasma jet isexcited in a rectangular waveguide, operated at a frequency of2.45 GHz, and is stabilized by the flowing channel of a workinggas through the nozzle. The charged particles, with a high kineticenergy produced from the plasma jet, caused a surface in a roughmorphology to be smooth, showing a change in the water contactangle. A scanning electron microscopy and a water contact-angleanalyzer characterized the samples treated in different gas mix-tures. The water contact angles of the samples were plotted asa function of the recovery time, showing that the sample treatedby Ar/H2 plasma jet has the smallest contact angle (∼ 3◦) andthe slowest recovery of the angle. In the estimation of the alu-minum-surface free energy, after the H2/Ar plasma treatment, thetotal surface free energy of the Al sample increased from 18.71 to82.81 mJ/m2 drastically.

Index Terms—Contact angle, metal, plasmas material-processing applications, surface treatment.

I. INTRODUCTION

SHEET METALS are generally coated with oils, for tempo-rary corrosion protection, or with lubricants, for a friction

reduction during the metal forming processes. These organiccoatings have to be removed before a further surface fin-ishing of the metal sheets [1]. Usually, aqueous cleaning isperformed in alkaline or acid treatment baths. However, thesewet-chemical treatments are problematic under environmentalconsiderations because of the disposal of harmful waste in thetreatment baths [2].

Plasma modification is one of the possible methods for theenhancement of the surface free energy, with many advantages[3]. Generally, plasmas used in dry etching, thin-film deposi-tion, and surface treatment for the display or semiconductorindustries are operated at low pressures. However, low-pressureprocessing is very costly due to the use of the vacuumequipment and vacuum components. The usage of vacuumprocessing also increases the fabrication cost and decreasesthe productivity [4]. The surface-treatment processing using

Manuscript received January 19, 2006; revised March 6, 2006. This workwas supported by the Korea Institute of Industrial Technology’s CleanerProduction Program under Grant 2004-A041-00.

D. H. Shin, C. U. Bang, J. H. Kim, Y. C. Hong, and H. S. Uhm are withthe Department of Molecular Science and Technology, Ajou University, Suwon443-749, Korea (e-mail: [email protected]).

D. K. Park and K. H. Kim are with Teus Company Ltd., Suwon 443-812,Korea.

Color versions of Figs. 1, 2, 4 and 5 are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2006.876486

atmospheric plasmas may reduce the processing costs andincrease the productivity. Over the past decades, many re-searchers have developed atmospheric-pressure glow dis-charges [5]–[9] for the various thin films and surface process-ing. For example, the dielectric barrier discharge (DBD), pulsedcorona plasma, radio-frequency (RF) plasma, etc., have beenstudied for the various applications of the surface treatmentof organic materials [10], [11], growth of organic thin films[12], [13], dry etching [14], etc.

Over the past several decades, research of the microwave-discharge technology has been in particular demand due toan industrial application of the microwave-generated plasmas.Low-pressure microwave plasmas have been used to modifypolymeric materials [15]–[17], because of their damage-freeprocessing. Atmospheric microwave plasmas generally producea high-temperature plasma flame due to ohmic heating by a highfrequency. For example, argon microwave plasma operated ata frequency of 2.45 GHz produces a high-temperature flameclose to about 3000 K at 5 cm from the waveguide [18], [19].However, the downstream flame of the microwave plasma inthe nozzle-type plasma jet can be used for surface modification,by cooling down the plasma-flame temperature drastically andblowing it out with open air at high gas flow rate. Moreover,there is no direct contact between the plasma and the nozzle,because the plasma jet is generated in the open air, at the tip ofnozzle. Therefore, the nozzle is relatively cool, and the problemassociated with the electrode corrosion is negligible. Electrons,ions, free radicals, and other molecular species inside theplasma flame play key roles in the surface-modification process.This paper reports the modification of the metal surfaces bymaking use of Ar, Ar/O2, and Ar/H2 microwave plasma jetsgenerated at atmospheric pressure, showing a change in thesurface roughness and a reduction of the water contact anglebefore and after plasma treatment.

II. EXPERIMENTAL APPARATUS AND METHOD

Fig. 1 shows a schematic diagram of the apparatus for theplasma modification making use of an atmospheric microwaveplasma jet. We previously reported the microwave plasma torch,of which a discharge tube was axially positioned across therectangular waveguide [20]. The microwave plasma torch hasbeen applied to the abatement of the CF4 emitted from thesemiconductor industries [21], decontamination of chemicaland biological agents [22], [23], synthesis of nano materialssuch as carbon nanotubes [24], titanium nitride [25], nitrogen-doped titanium dioxide [26], [27], iron oxide [28], etc. In

0093-3813/$20.00 © 2006 IEEE

1242 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 4, AUGUST 2006

Fig. 1. Schematic diagram of the apparatus for plasma modification, makinguse of an atmospheric microwave plasma jet.

this paper, as shown in Fig. 1, the plasma jet consists of a2.45-GHz magnetron, a circulator, a matching network (3-stubtuner and directional coupler), a tapered waveguide, and aplasma nozzle. The microwave radiation generated from themagnetron passes through the circulator, and the matchingnetwork is guided through a tapered waveguide and induces theintensive electric field on the tip of the plasma nozzle, which isplaced in an aperture installed at one-quarter wavelength fromthe short end of the tapered waveguide. The plasma initiation isaccomplished by touching the tip of the nozzle with a metal rod,which provides the initial electrons for the plasma generation.As mentioned earlier, the microwave plasma torch producesa high-temperature plasma flame of about 3000 K [18], [19].Therefore, the nozzle-type microwave plasma jet was used forthe metal surface modification, by cooling down the plasma-flame temperature drastically. The plasma power for all theexperimental conditions in the experiment was 0.6 kW.

Fig. 2 is a photograph of the microwave plasma jet inoperation, showing a narrow plasma column and an intenseplasma radiation. An external part of an MP3 player composedof aluminum was used as a sample for the plasma surfacemodification. The sample was placed on a holder of a samplemoving system in the direction of the plasma downstream andwas located at 2 cm from the tip of the plasma nozzle. Alsamples were cleaned in an ethanol solution and dried in air,before the plasma treatment. All the samples in the experimentwere treated at a speed of 0.9 cm/s, by the automatic movingsystem. The tests for the surface modification were carriedout with different gas compositions. The surface morphologiesof the samples before and after the plasma treatment wereobserved by employing a scanning electron microscope (SEM).The contact-angle analyzer was also used to investigate thechanges of the water contact angles, along with aging timesand plasma gas compositions. A drop of water (20 µL) wassuspended on the Al samples and was captured by a digitalcamera, and, then, the water contact angle was measured alongwith the aging time.

III. RESULTS AND DISCUSSION

The Al-sample surfaces were exposed to the Ar, Ar/O2, andAr/H2 plasmas to investigate the effects of the plasmas onthe surface modification. Fig. 3 shows the SEM images of the

Fig. 2. Photograph of the microwave plasma jet, in operation, for the treat-ment of the Al sample.

Fig. 3. SEM images showing the surface morphologies of the Al samplestreated by the plasma jets with the different gas compositions. (a) Raw samplewith many scratches on the surface and (b)–(d) samples treated by the plasmajets of the 10-Lpm Ar, 10-Lpm Ar/0.1-Lpm O2, and 10-Lpm Ar/0.1-Lpm H2,respectively.

surface morphologies of the Al samples treated by the plasmajets generated by the different gas compositions. Fig. 3(a)is the SEM image of the raw sample with many scratcheson the surface. Fig. 3(b)–(d) shows the SEM images of thesamples treated by the plasma jets of 10-Lpm Ar, 10-Lpm Ar/0.1-Lpm O2, and 10-Lpm Ar/0.1-Lpm H2, respectively. Theinner diameter of the tungsten nozzle used in the experimentis 1.2 mm, and, then, the flow velocity of the Ar, as a workinggas at the nozzle tip, is about 147 m/s. Therefore, the plasma-flame temperature is cooling down drastically, and the energeticions produced in a high gas flow velocity bombard the scratchedsurface. As shown in Fig. 3(b)–(d), one can identify thatthe surface roughness of the plasma-treated samples has beendecreased apparently, although the difference of the averagesurface morphologies among them were not measured. In thiscontext, we measured the water contact angles of the samplestreated with the different plasma gases. The wettability of thesolid surface depends on the surface energy and the geometricalstructure of the solid surface [29]. Generally, the water contactangle decreases, as the surface roughness decreases.

SHIN et al.: TREATMENT OF METAL SURFACE BY ATMOSPHERIC MICROWAVE PLASMA JET 1243

Fig. 4. Shapes and contact angles of the water drops placed on the samplestreated by the plasma jets with the different O2 flow rates. (a) Raw sample;(b) sample treated by the 10-Lpm Ar/0-Lpm O2; (c) sample treated bythe 10-Lpm Ar/0.1-Lpm O2; and (d) sample treated by the 10-Lpm Ar/0.15-Lpm O2.

Fig. 5. Shapes and contact angles of the water drops placed on the samplestreated by the plasma jets with the different H2 flow rates. (a) and (b) arethe samples treated by the 10-Lpm Ar/0.1-Lpm H2 and by the 10-Lpm Ar/0.15-Lpm H2, respectively.

Fig. 4 shows the shapes and the contact angles of the waterdrops placed on the samples treated by the plasma jets with dif-ferent O2 flow rates in a 10-Lpm Ar base gas. The raw samplein Fig. 4(a) has a water contact angle of about 90◦, revealing ahydrophobic surface. When the O2 flow rates in the 10-Lpm Arbase gas is increased from 0 to 0.15 Lpm, the contact angleis measured to be 22◦, 11◦, and 14◦, respectively, as shownin Fig. 4(c)–(d). Regardless of the O2 contents in the plasmagas, the water contact angle of the samples exposed to theplasma jet decreases apparently. The result illustrates that thehydrophilicity of the Al-sample surfaces exposed by the plasmajets increases profoundly. The hydrophilicity increase is due tothe decrease of the roughness, as shown in Fig. 3, and the gainof the Ox groups on the Al-sample surface during the plasma-mediated oxidation [30], although the chemical composition onthe Al-sample surface was not analyzed. However, the furtherincrease of the O2 flow rate from 0.1 to 0.15 Lpm increased thecontact angle slightly. Generally, the natural oxidation of theAl metal can make the layer of the oxide reach up to 10 nm,meaning that the Al surface is already saturated with oxides.The treatment with the O2 microwave plasma jet increases thetemperature of the Al surface and the diffusion of the oxide intothe sublayer. Therefore, we suggest that, in case of O2 additionto the Ar base gas, the O2 flow rate more or less than 0.1 Lpm

Fig. 6. Contact angles as a function of the elapse time, after the plasmatreatment. The rectangles, triangles, and circles correspond to the samplestreated by the 10-Lpm Ar, 10-Lpm Ar/0.15-Lpm O2, and 10-Lpm Ar/0.15-Lpm H2 plasmas, respectively.

Fig. 7. Contact angles as a function of the H2 flow rate in a H2/Ar mixture ata 30-min elapse time, after the plasma treatment. The 10-Lpm Ar was used asa base gas, and the applied microwave power was 0.6 kW.

could be used for the Al-surface cleaning for our experimentalconditions.

Fig. 5 shows the contact angles and the shapes of the waterdrops on the Al-sample surfaces exposed by: 1) H2 0.1 Lpm and2) H2 0.15 Lpm in a 10-Lpm Ar base, respectively. As observedin Fig. 4(a) and (b), the raw sample and the sample treated bythe 10-Lpm Ar plasma jet have the contact angle of 90◦ and22◦, respectively. Unlike Fig. 4(c) and (d), as the H2 flow ratesincrease from 0.1 to 0.15 Lpm, the contact angles decrease from9◦ to 3◦, showing a super hydrophilicity (0◦–10◦) [31]. Fromthe viewpoint of the contact angle, the H2/Ar plasma was moreeffective for the modification than the O2/Ar plasma.

We have observed that, if allowed to react with ambient air,the plasma-treated surface recovers its hydrophobic character,revealing its contact-angle hysteresis. Maintaining a high sur-face energy for a long time is very important for the industrialapplications. In this context, the aging property of the surface


TABLE ISURFACE-ENERGY PARAMETERS OF THE PROBE LIQUIDS USED IN THIS EXPERIMENT AND THE CONTACT ANGLES MEASURED EXPERIMENTALLY

has an immediate connection with the production cost andstorage [32]. Fig. 6 shows the contact angles as a functionof the elapse time. The rectangles, triangles, and circles inFig. 6 correspond to the samples treated by the 10-Lpm Ar,10-Lpm Ar/0.15-Lpm O2, and 10-Lpm Ar/0.15-Lpm H2 plas-mas, respectively. All of the samples have the contact angle of90◦ and under 20◦, before and after plasma treatment, respec-tively. Interestingly, three samples in Fig. 6 have apparentlydifferent recovery time for their saturated contact angles. Thesamples treated by the 10-Lpm Ar, 10-Lpm Ar/0.15-Lpm O2,and 10-Lpm Ar/0.15-Lpm H2 plasmas require approximately30, 60, and 90 min, respectively, for their saturated contactangles. Also, we observe from Fig. 6 that all of the Al samplesnever fully regain its initial hydrophobicity, showing the so-called hysteresis. As shown in Fig. 6, the H2 is more effectivethan the O2 in the improvement of the surface wettability, andthe effect also lasts longer. In this regard, we measured thecontact angles along with the H2 flow rate in the plasma gas.

Fig. 7 shows the contact angles in terms of the H2 flow rateat a mixture of the plasma gas. The data in Fig. 7 were obtainedat a 30-min elapse time, after the plasma treatment. The rawsample before the treatment has the contact angle of about 90◦.As shown in Fig. 7, as the H2 flow rates increase from 0 to0.3 Lpm, the contact angles gradually decrease from 35◦ to 16◦,revealing the improvement of the Al-surface wettability. Thecontact angle in the H2 flow rate more than 0.3 Lpm seems tobe saturated. Generally, because the Al surface is covered witha layer of the native oxide, treatment with the O2 plasma canonly deposit the oxygen atoms on the surface. However, the H2

plasma can react chemically with the native-oxide layer and cangraft the OH radicals permanently. For example, the previousliteratures [33], [34] presenting the plasma treatment of the Alsurface showed the existence of aluminum hydroxides such asAl(OH)3, AlO(OH), etc. Therefore, the increased wettability isbelieved to result from the OH groups on the Al surface.

Further insights into the interactions between the liquid andsolid surfaces are demonstrated in terms of the dispersive andpolar-energy contributions in the equation of Owens and Wendt[35]. This is a linear equation, Y = mX + b, in which theslope m and the intercept b are given by the square root ofthe polar and dispersive components of the solid-surface freeenergy [(γp

s )1/2 and (γds )1/2, respectively], such as

Y =γl(1 + cos θ)

2√

γdl

=√

γps X +

√γds (1)

where X = (γpl )1/2/(γd

l )1/2. The X values are given as X =0.805 for polyethylene-glycol (PEG), 0.94 for glycerol, and1.53 for water. We assume that the total polar components and

Fig. 8. Owens and Wendt plot for the Al surface according to (1). Eachdata point is an average of five repeat measurements. The error bars show thestandard deviation of each data point.

the dispersive of the liquid-surface free energy (γl, γpl , and γd

l ,respectively) are known, and the contact angles are measured,as shown in Table I. Fig. 8 shows a plot according to (1), usingthe contact angles of the three testing liquids measured in thispaper. Each data point in Fig. 8, for the contact angle, is an av-erage of more than five repeat measurements. We estimated thepolar and the dispersive components of the surface tension ofthe Al samples from the slope and the intercept of the linear fitin Fig. 8. The total surface energy, γp

s + γds , for the Al sample,

before the plasma treatment, is calculated to be 18.71 mJ/m2

(γps = 10.18 mJ/m2, γd

s = 8.53 mJ/m2). However, the totalsurface energy after plasma treatment increased drastically to80.18 mJ/m2 (γp

s = 75.69 mJ/m2, γds = 4.49 mJ/m2), for the

O2/Ar plasma-treated Al sample, and to 82.81 mJ/m2 (γps =

78.32 mJ/m2, γds = 4.49 mJ/m2), for the H2/Ar plasma-treated

Al sample. The polar energy of the untreated Al sample is muchless than the plasma-treated Al sample. Such increased polarinteractions could explain why the O2/Ar or H2/Ar plasma-treated Al samples are more hydrophilic than the untreated Alsample, comparing the interactions of the Al surface and theliquids, as shown in Figs. 4–6.

Finally, the active species from the plasma bombard andreact with the surface of the materials to change their surfaceproperties either temporarily or permanently, introducing themodification of the rough surface and the chemical functionalgroups [36], as mentioned in Figs. 3 and 6, respectively. Asshown in Fig. 8, these physical and chemical effects on thesurface also change the surface free energy of the Al, increasingthe hydrophilicity.

SHIN et al.: TREATMENT OF METAL SURFACE BY ATMOSPHERIC MICROWAVE PLASMA JET 1245

IV. CONCLUSION

This paper reported the surface modification of the Al-sample plasmas treated with the different gas compositions.The atmospheric microwave plasma jet with a narrow plasmacolumn and a gas flow velocity of 147 m/s may be suitable forthe sample with a narrow space. As shown in Fig. 3, the plasmajet removed many scratches on the surface, bombarding on thesample surface with the energetic plasma species. The contact-angle measurement indicates that the H2/Ar plasma was moreeffective for the modification than the Ar and O2/Ar plasmas,showing the lowest contact angle (3◦) after plasma treatment.The H2/Ar plasma-treated sample also showed the lowest sat-uration contact angle, which is important for the maintenancefor a long time with the high surface energy because it hasan immediate connection with the production cost and stor-age. After the H2/Ar plasma treatment, the total surface freeenergy of the Al sample increased from 18.71 to 82.81 mJ/m2

drastically, revealing the improvement of the surface wettabil-ity. The samples with a super hydrophilic surface may improvethe adhesion between the Al surface and the painting layer.In this context, the microwave plasma jet is applicable to thetreatment and adhesion-promotion of other metal surfaces.

ACKNOWLEDGMENT

The authors would like to thank D. W. Lee at the Centerfor Materials Characterization and Machining, Ajou University,Suwon, Korea, for his help in obtaining the SEM data.

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Dong H. Shin was born in Seoul, Korea, on October6, 1977. He received the B.S. degree in physics fromAjou University, Suwon, Korea, in 2004, where he iscurrently working toward the M.S. degree in appliedphysics.

His research interests include applications of theatmospheric-pressure microwave plasma torch.

Chan U. Bang was born in Daegu, Korea, on De-cember 6, 1977. He received the B.S. degree inphysics from Ajou University, Suwon, Korea, in2003, where he is currently working toward the M.S.degree in applied physics.


Jong H. Kim was born in Seoul, Korea, on March3, 1980. He received the B.S. degree in physics fromAjou University, Suwon, Korea, in 2005, where he iscurrently working toward the M.S. degree in appliedphysics.


Yong C. Hong was born in Chungsun, Korea, on July26, 1973. He received the Ph.D. degree in plasma sci-ence from Ajou University, Suwon, Korea, in 2005.

Currently, he is a Postdoctoral Researcher at AjouUniversity. His research interests include the physicsand applications of the atmospheric-pressure mi-crowave plasma torch, including elimination of thechemical and biological warfare agents, abatement ofthe fluorinated compound gases, synthesis of nano-materials, and generation and diagnostics of large-volume plasma at low pressure.

Han S. Uhm (A’84–M’87–SM’87) received thePh.D. degree in physics from the University of Mary-land, College Park, in 1976.

He was with the U.S. Naval Surface WarfareCenter from 1978 to 1999. He is a Professor atAjou University, Suwon, Korea. The major re-search areas where he is recognized as a technicalexpert are charged particle beams (charged particle-beam propagation in the air, beam properties in theparticle accelerators, collective ion acceleration, andhigh-power electrical-pulse systems); high-power

microwave generation (gyrotron, magnetron, free-electron laser, and klystron);plasma material processing (large-volume plasma generation for plasmaprocessing, focused ion-beam generation, and plasma ion implantation for thewear- and corrosion-resistant materials); neutron transport physics (neutronradiography and shallow-water mine detection); plasma chemistry (plasma-assisted chemical-vapor elimination and air purification); and atmospheric-plasma generation and applications.

Dae K. Park received the B.S. degree in metalengineering from Korea University, Seoul, Korea,in 1996.

He is currently a Research Manager with TeusCompany Ltd., Suwon, Korea.

Ki H. Kim received the M.S. degree in businessadministration from Ajou University, Suwon, Korea,in 1991.

He is currently Chief Executive Officer of TeusCompany Ltd., Suwon.

treatment of metal surface by atmospheric microwave plasma jet

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