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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.
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