Surface modification for hydrophilic property of stainless steel ...

Surface and Coatings Technology 171 (2003) 312–316Surface modification for hydrophilic property of stainless steel treated byatmospheric-pressure plasma jetM.C. Kim*, D.K. Song , H.S. Shin , S.-H. Baeg , G.S. Kim , J.-H. Boo , J.G. Han , S.H. Yanga,b, a a a a b b aaPlasma Technology Application Division, Korea Institute of Industrial Technology, Cheonan 330-825, South KoreabCenter for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon 440-746, South KoreaAbstractSurface of a stainless steel has been modified by atmospheric-pressure plasma jet method at room temperature. The impulsevoltage is applied to ignite a plasma discharge using high purity (99.999%) reactive gases: N2 and O 2. The treated stainless steelis characterized by the activation property of the surface using a contact angle analyzer. Surface energy for the treated stainlesssteel is increased remarkably when compared to the untreated surface. From the results of X-ray photoelectron spectroscopy andatomic force microscopy, we could confirm that the main functional groups, causing the change in hydrophilic surface weregenerated under the surface reactions caused by reactive etching and oxidation of ions and activated species in the plasma. Inaddition, the aging effect during the duration of the hydrophilic property is also studied to investigate the production cost for theindustrial applications. 2003 Elsevier Science B.V. All rights reserved.Keywords:Surface modification; Atmospheric pressure; Hydrophilic property1. IntroductionIn the early 1990s, the study in applications ofatmospheric plasma treatment has been graduallyincreased due to its low cost and flexibility of acontinuous process. Generally, low-pressure plasma haswide applications in materials processing but there areseveral disadvantages. Vacuum system is expensive andrequires heavy maintenance. Besides, the size of theobject that could be treated is limited by the size of thevacuum chamber. Atmospheric-pressure plasma overcomesthe drawback of vacuum processes. Among manyatmospheric-pressure plasma sources, the plasma jetsystem is distinctively more efficient than others, sinceits thermal temperature is low, and charged particle andreactive species density is high in the plasma w1x.Plasma treatment for surface modification is used toproduce the hydrophobic or hydrophilic surfaces onmetals, plastics, glasses or polymers. Recently, plasmapre-treated metal products have new applications in thefield of automobile painting, printed circuit board manufacturing,and electromagnetic interference shieldingmaterials, etc. w2x. In these fields, a surface modificationis used to reveal high degree of adhesion property*Corresponding author.E-mail address: mckim1973@hanmail.net (M.C. Kim).between the different kind of materials, even in thecondition of water-based adhesives and paints, whichaffects environmental non-pollution. Therefore, the surfacescould be activated for the adhesion enhancementon the metal–polymer or metal–metal combinations byatmospheric-pressure plasma jet (APPJ) system. However,there seems to be no established basic concept toexplain the surface phenomena for hydrophilic property.Therefore, this paper reports the new application ofthe atmospheric plasma to the continuous surface modificationfor a hydrophilic process. The purpose of thisstudy is to obtain the optimum conditions of hydrophilicsurface using our APPJ system and to confirm thesurface morphology and functional species.In order to understand the chemical reactions andphenomena on the stainless steel, X-ray photoelectronspectroscopy (XPS) and atomic force microscopy(AFM) methods are used for the treated surfaces in theoptimum conditions. The surface energy is measured bythe contact angle analyzer, which is also adopted inanalysis of aging characteristics.2. ExperimentalAPPJ experiments are carried out using the atmosphericplasma apparatus made by Agrodyn Plasma Treat0257-8972/03/$- see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00292-5

M.C. Kim et al. / Surface and Coatings Technology 171 (2003) 312–316313In comparison, the steel products normally have a roughsurface. All the treatments are carried out under theroom temperature. The aging tests are performed in theconstant conditions of the relative humidity of 64% for4 days.3. Results and discussion3.1. Contact angle analysis and optimum condition forhydrophilic propertyAs a first step of our analysis, we performed contactangle analysis to obtain the optimized conditions for ahydrophilic surface with respect to the nozzle-to-surfaceFig. 1. Contact angle data under different nozzle velocity (a) and gapdistance (b). Contact angle images (c) before treatment and aftertreatment in the optimum conditions of NSG 5 mm and NMV of 10mmys.GmbH, Bielefeld, Germany. The power for the plasmageneration and sustenance is supplied with the impulsevoltage into the jet-nozzle electrode via the transformer.The maximum voltage is applied into the jet-nozzleelectrode. The output frequency is approximately 10 kVfrom peak to peak at the frequency of 16–20 kHz. Thelength of plasma jet flame is approximately 30 mm andthe diameter is 10 mm. Mixed N and O with a high2 2degree of purity (99.999%) are used as reactive gaseswith the ratio of N :O s4:1 similar to air. The gases2 2are controlled by manual flow-controller and constantlyflowed into the jet-nozzle for a plasma ignition. The jetnozzlebody is fixed on the frame of the X–Y movingtable made to treat large-area substrate such as web orpolymer films. The stainless steel is sequentially cleanedin acetone, methyl alcohol, and de-ionized water byultrasonic cleaning method. For clearer confirmation onAFM results, silicon (100) wafer is also cleaned witha fore-mentioned method and treated in the plasmaunder the same conditions due to its highly flat surface.Fig. 2. (a) High-resolution XPS analysis of C ls (a), Ols(b), andFe 2p on the same surface as Fig. 1c.3y2

314 M.C. Kim et al. / Surface and Coatings Technology 171 (2003) 312–316Table 1Surface compositions of plasma treated surfaces by XPS analysisat.% C O Fe CrUntreated SUS 41.6 47.8 4.5 6.1Treated SUS 24.7 59.3 16 0gap (NSG) and nozzle velocity. According to the numericalsimulation of the concentrations of species by Jeongw1x, the effluent of the plasma jet depends on thefunction of distance between nozzle and surface. Fromthis dependency, we could realize that the concentrationsof activated species like O and N atoms as well as themetastable oxygen and nitrogen molecules in our studyshow non-equilibrium distribution. The plasma jet ispartially under the condition of atmospheric pressure. Inaddition, the efficiency of surface modification is alsofollowed by the treatment time per unit area on thesurface and controlled by the moving velocity of thenozzle. Fig. 1a–c shows the optimum conditions andcontact angle images for the surface activation withincreasing the NSG distance and the nozzle movingvelocity (NMV) using the reactive mixed gases in theratio of N 2:O2s4:1. As the figures indicate, the contactangles are increased with increasing the NSG and NMVand it gives a good account for the theory as mentionedabove. The best surface energy calculated from thecontact angle value is approximately 71.49 mNym inthe conditions of 10 mm (NSG) and5mmys (NMV).Below the conditions of 10 mm (NSG) and5mmys(NMV), the treated steel substrates are damaged due tothe plasma heat and arc generated in the end of nozzletip. Hence it is appropriate to neglect the values belowthat conditions.3.2. The analysis of surface activation by X-ray photoelectronspectroscopyIn the optimum conditions obtained from the contactangle data, the steel sheets with 0.2 mm thickness aretreated using the mixed gases of N 2:O2s4:1. Thesubstrate is compared with the untreated steel. Theresults of these measurements are shown in Fig. 2. Fig.2a indicates the presence of C 1s core-level XPS spectraobtained from the stainless steels after the treatment. Itis considered that the carbon is caused by the impuritycontained steel substrates and the air contamination suchas CO2and CO molecules, which could be broken topieces or excited to metastable states by heat andcollisions in the plasma jet. From the curve fitting ofFig. 3. The section (a) and three dimensional morphology (c) images of untreated stainless steel surface. After treating, the section (b) and threedimensional morphology (d) images by AFM.

M.C. Kim et al. / Surface and Coatings Technology 171 (2003) 312–316315C_ O, C–C, C–O bonds, the adsorption and formationof new functional groups can be shown, which couldsupport a hydrophilic surface w3–5x, In the cases of O1s core-level spectra in Fig. 2b, several chemical bindingstates are also observed in the range of 530–533 eV,indicating the binding energy of the Fe2O 3, FeOOH, andHOw6x. 2 Fe 2p3y2core-level spectra are shown in Fig.2c, which has a connection with the decomposition ofO 1s peak w6x. Thus it is clear that the surface oxidationis occurred occasionally by O atoms, O2molecules, andthe other activated species including a carbon on thesubstrate and additionally new functional groups forhydrophilic surfaces were generated. Under the presentresults, the nitrogen elements with the types of atomsand metastable species are absent on the steel surfaces.It can be interpreted by collision cross-sections between2 2N 2 (0.43 nm ) and O 2 (0.40 nm ), and highly chemicalactivity with three bonds in the atomic phase on thenitrogen gas. It could be inferred from these reasonsthat the amount of dissociated N atoms could be largerthan the oxygen atoms, and the atoms and metastablespecies are combined with nearby N, O, C, andyoractivated molecules before arriving at surface. From theresults of Table 1, it could be expected that the nitrogenspecies activated could easily arrive and bombard thesurface, leading to the effects of sputtering and reactiveetching. On the other hand, it is considered that oxygenspecies activated are in the role of surface oxidation,affecting hydrophilic property. Therefore, two reactionsoccurred to increase contact angle on surface; one is thecleaning effect by nitrogen atoms and activated species.The other one is surface oxidation by oxygen atoms andactivated species. These results could also be explainedby a topography and section analysis using AFM.3.3. Analysis of surface and section morphology byatomic force microscopyFig. 3a, b and c, d show the section images and thethree-dimensional morphologies of both untreated andtreated substrates in optimized conditions. After thetreatment, the new grains appeared densely in comparisonwith the untreated morphology. Besides the surfaceis also modified into sharp and the height of prominencewere decreased as a result from the section analysis. Inaddition, the topographic properties are analyzed usingthe treated silicon (100) wafer as a substrate in thesame conditions due to its advantage of flatness. Theresults are shown in the Fig. 4a and c. These figuresclearly show that the stainless steel and silicon surfacesare bombarded and etched by nitrogen molecules mainly,but, at the same time, partially the oxide grains containedcarbon are grown on the surface. In this respect, theobtained results are corresponded to interpretation ofXPS data. To confirm the effects of mixed gases ratio,we also have treated the silicon substrate in the conditionof N 2:O2s1:4 and showed in Fig. 4b and d. Thesefigures reveal that the grain growth is increased withFig. 4. The section (a) and three dimensional morphology (c) images of treated substrate in the condition of N :O s4:l on silicon (100) wafer.2 2The section (b) and three dimensional morphology (d) images of silicon (100) wafer in the condition of N :O s1:4 by AFM analysis.2 2

316 M.C. Kim et al. / Surface and Coatings Technology 171 (2003) 312–316Fig. 5. Contact angle data obtained under different exposed time during 4 days of the treated surface by APPJ in the optimum conditions.increasing the amount of oxygen gas and the bombardingand etching reactions are decreased with decreasing thenitrogen amount. The results of contact angle analysisof treated substrates are very similar under the treatmentconditions of N :O s4:1 and N :O s1:4 on steel and2 2 2 2silicon. In the case of stainless steel, the contact anglesare us15.07 and 14.05, respectively, in the conditionsof N :O s4:1 and N :O s1:4. Both results of silicon2 2 2 2substrates are also very similar as below us10. Thisresult could be interpreted in the following way; thesurface activation was governed by the effects of bombardingand etching reactions of nitrogen molecules inthe condition of N 2:O2s4:1, to the contrary, the effectsof grain growth composed to oxide contained carbon byO atoms, O , O , Oof N 2:O2sl:4 mainly w1x., H O and CO in the conditionq y 2q2 23.4. Aging test for application to industrial processIn the industrial applications, the aging property isimportant for the maintenance during the long time withthe high surface energy because it has a deep withproduction cost and storage. Therefore, we have measuredcontact angles for 4 days using the substratestreated in the optimized conditions as mentioned aboveat room temperature and relative humidity of 64%. Fig.5 shows that the surface energy is decreased within 15h with respect to the increase in exposed time. Therefore,the highly activated surfaces could react rapidly evenwith stable molecules due to its high surface energiesas discussed in XPS and AFM results.4. ConclusionsThe stainless steel substrates was successfully modifiedinto hydrophilic surfaces using mixed gases ofnitrogen and oxygen by APPJ method. The highestsurface energy for adhesion enhancement was 71.49mNym at the NSG of 5 mm in the condition of NMVof 10 mmys, respectively. The results of XPS data andAFM analysis reveal that activated nitrogen and oxygenspecies could etch and oxidize the surface, respectively,resulting in the increased surface energies. The agingtest results show that the highly activated surface isobtained and has a short duration time of approximately15 h in air condition after the treatments. In conclusionwe could understand the surface phenomena and confirmthat the stainless steel could be easily treated to hydrophilicproperty under the air condition by plasma jetmethod.Referencesw1x A. Schutze, J.Y. Jeong, S.E. Babayan, J. Park, G.S. Sewyn,R.F. Hicks, IEEE Trans. Plasma Sci. 26 (1998) 1685.w2x O. Goossens, E. Dekempeneer, D. Vangeneugden, R.Van de Leest, C. Leys, Surf. Coat. Technol. 142–144 (2001)474.w3x I. Olefjord, L. Wegrelius, Corros. Sci. 38 (1996) 1208.w4x C. Duret-Thual, D. Costa, W.P. Yang, P. Marcus, Corros. Sci.39 (1997) 927.w5x A. Claes-Olof, Olsson Corros. Sci. 37 (1995) 473.w6x A. Pereira, A. Cros, Ph. Delaporte, W. Marine, M. Sentis,Appl. Surf. Sci. 8121 (2002) 3.